This invention relates generally to the manufacture of thin, flexible tubing and specifically, to the manufacture of flexible drip hose or tape used in agricultural irrigation.
Drip irrigation hose or tape has been available for several years. Typically, agricultural drip tapes are formed from relatively thin, flexible, continuous plastic strips folded over and seamed along a longitudinal edge to establish a primary flow path. One or more secondary flow paths are typically formed within the primary flow path by fixing discrete emitter devices along the length of the tape or hose, or by applying parallel strips of plastic material within the hose interior (for example, in the area of the longitudinal edge overlap) to form a secondary flow path. Generally, the primary flow path is connected to the water supply with inlets and outlets to and from the secondary flow path, so that water flows from the primary path to the secondary flow path, and then out of the secondary path outlets in a controlled fashion. Some tape or hose constructions incorporate turbulence inducing regions in the secondary flow path to prevent clogging and reduce the sensitivity of the flow rate to pressure changes.
Drip irrigation hoses or tapes are well represented in the patent literature, and examples may be found in U.S. Pat. Nos. 3,870,236; 3,896,999; 4,009,832; 4,247,051; 4,430,020; 4,473,191; 4,874,132; 4,880,167; 4,984,739; 5,163,622; 5,181,532; 5,203,503; 5,207,386; 5,282,578; 5,333,793; and 5,688,072.
Despite the wealth of innovative drip irrigation technology, significant areas of concern remain relating to reliability and cost. For drip tape to be effective and commercially viable, it is essential that the secondary flow path not become clogged with solid matter in the water supply, or by outside debris blocking the outlets. At the same time, drip tape must be economical to manufacture.
In commonly owned U.S. Pat. No. 6,120,634 issued Sep. 9, 1997, there is disclosed a new and improved drip tape formed by a strip of extruded flexible polyethylene (PE) material folded over, heated and seamed along overlapped longitudinal edges. Interiorly of the tape, and in an area remote from the overlapped seam, there is a pre-formed longitudinally extending plastic bead or strip which defines a series of axially spaced secondary flow paths. The secondary flow path bead or strip is extruded onto a heated portion of the tape, prior to seaming.
As described in the ""634 patent (the entirety of which is incorporated herein by reference), each secondary flow path has a secondary flow channel including inlet, turbulence inducing, and outlet regions, all of which are preformed on one side of the hot melt bead or strip. The pattern side of the bead is applied face down on the sheet so that the sheet wall itself closes the secondary flow channel except for a plurality of inlets formed in the bead at longitudinally spaced locations along the inlet region. These inlets are arranged perpendicular to the longitudinal axis of the tape, and thus also perpendicular to the secondary flow channel.
The inlet region leads to a turbulence inducing region formed by a series of peaks and valleys on opposite sides of the secondary flow channel, in longitudinally offset relationship so that the peaks on one side of the secondary flow channel in this region project towards the valleys on the opposite side of the flow channel.
Downstream of the turbulence inducing region, an outlet region is provided which communicates with an elongated slit in the tape wall which allows the water in the secondary flow path to escape in a controlled drip-like fashion. The outlet region, or reservoir, is otherwise axially closed in the downstream direction, thus isolating the path from the inlet region of the next adjacent downstream secondary flow path, and thus also forcing all water to exit via the elongated slit in the tape wall.
The manufacturing process as described in the ""634 patent has proven to be generally satisfactory, but faster production speeds are desired.
This invention relates to an improved process for seaming overlapped edges of a flexible strip or web that forms the primary flow path in an agricultural drip irrigation tape. In addition, the invention relates to facilitating the adhesion of a pre-formed, secondary flow path strip within the interior of the tube. The preferred utilization of focused infrared heating, combined with other measures to facilitate bonding of both the secondary flow path to the tape and the overlapped marginal edges of the tape, have resulted in significantly greater production speeds.
In one exemplary embodiment, the flexible strip or web is comprised of a blend of high and linear low density polyethylene material (HDPE and LLDPE, respectively) continuously extruded from a first extrusion head or die. Simultaneously, LLDPE material is co-extruded from a second and third extruder onto upper and lower surfaces of respective marginal edges of the web of material. By co-extruding the LLDPE material along the marginal edges, bonding of subsequently overlapped marginal edges is facilitated, particularly at high production speeds. In addition, flow from a fourth extruder is co-extruded onto a center area of the upper surface of the web of material. This narrow center strip is also comprised of LLDPE material, and facilitates the adhesion of a pre-formed secondary flow path strip to the web. Because of the molten state of the extrudates, all three co-extruded strips are bonded or welded to the web, and the marginal edge strips and center strip are sufficiently thin that they are substantially flush with the web.
In a second exemplary embodiment, the additional marginal and center area extrusions of LLDPE material may be omitted. In other words, the marginal edges remain the blended material from which the web is made, and the secondary flow path is bonded directly to the web without the aid of an intermediate center strip. This second embodiment is viable particularly at lower production speeds.
In both embodiments, it is preferred that heat be applied to the web along the center area, where the secondary flow path strip is to be bonded to the web, and to the marginal edges of the web just prior to the folding and seaming, by means of focused infrared heating.
It will be appreciated that the co-extrusion of narrow strips onto the marginal edges of the web to facilitate bonding or seaming of the edges can be performed independently of the manner in which secondary flow paths (or even discrete flow emitters) are added to the tape. Conversely, the co-extrusion of a sealing or bonding strip onto the center area of the web to facilitate bonding of a secondary flow path strip can be done independently of the manner in which the overlapped marginal edges of the web are seamed.
In its broader aspects, therefore, the present invention relates to a method of forming a tubular irrigation drip tape comprising a) extruding an elongated, continuous web of polymeric resin material; b) heating marginal edges of the web by exposure to a focused infrared heating unit; c) folding the web such that the marginal edges of the web are overlapped; and d) pressing the overlapped marginal edges to thereby bond the overlapped marginal edges together.
In another aspect, the invention relates to a method of forming tubular irrigation drip tape having a primary flow path and a secondary flow path comprising:
a) extruding a web of material having a pair of longitudinally extending, substantially parallel marginal edges;
b) heating a center area of the web between and substantially parallel to the marginal edges utilizing a first focused infrared heating device;
c) applying one or more secondary flow path components to the center area;
d) heating the marginal edges, utilizing a second focused infrared heating device;
e) folding the web such that the marginal edges overlap to form a seam; and
f) applying pressure to the seam.
In still another aspect, the invention relates to a method of forming a tubular irrigation drip tape comprising:
a) coextruding an elongated continuous web of one material, with marginal edges of another material;
b) extruding a center strip of material along a center area of the web, between the marginal edges;
c) heating the center strip and depositing a secondary flow path strip onto the center strip;
d) applying pressure to the center strip and the secondary flow path strip to thereby bond the secondary flow path strip to the web;
e) heating the marginal edges;
f) folding the web to cause the marginal edges to overlap; and
g) applying pressure to the overlapped marginal edges.
Additional advantages of the disclosed invention will become apparent from the detailed description that follows.